ORBITAL ANOMALIES OF THE PERIODIC COMETS BRORSEN, FINLAY, AND SCHWASSMANN-WACHMANN-2

Activation of new discrete sources appears to be a common phenomenon o n the nuclei of periodic comets. Some of these sources have life spans as short as 1-2 revolutions about the Sun or less, while others are m uch more persistent (enduring for dozens of revolutions or longer). On e line of evidence for an episode of this kind is an orbital anomaly, a sudden redistribution of the momentum that is transferred to the nuc leus by the sublimating mass. Such discontinuities in the orbital moti on have been reported for a number of periodic comets, of which Brorse n, Finlay, and Schwassmann-Wachmann 2 are investigated in some detail in this paper. Since the ''ignition'' and/or deactivation of a discret e source also entail parallel changes in the comet's water production and light curves, the latter provide constraints on the activation/dea ctivation scenarios. Although no unique solution can be offered in any particular case, the best conditions for detecting such events genera lly occur when the nucleus rotation vector is near the orbital plane a nd normal to the Sun's direction at perihelion and when the activated/ deactivated source is near one of the rotation poles. Especially for h ighly irregular nuclei, forced precession is a likely byproduct of suc h episodes and it may assist in activating additional sources by alter ing the insolation distribution over the nucleus surface.